The photoelectrochemical (PEC) wet etching technique was used to etch semi-insulating-GaN epitaxial layers grown by plasma-assisted MBE on sapphire substrates. Systematic studies were conducted to optimize the etching parameters such as the light intensity and concentration of the solution. It is found that the optimal concentration of the etching solution consisted of 0.1M KOH: 0.05M K₂S₂O₈ in proportion of 1:1. The film was etched under the illumination of the He-Cd laser at the light intensity of 1.68W/cm². This gives etching rate of about 22nm/min and the root-mean squared (RMS) roughness of the etched surface is 17.38nm. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) measurement results indicate that the etched surface is comparable with the as-grown GaN films, and can be further improved by the ultrasonic treatment in hot KOH solution.To investigate the effects of PEC wet etching on the electronic properties of the films, we fabricated cross-bridge Al₀.₁₃Ga₀.₈₇N | GaN heterostructures under the different etching conditions and ultrasonic treatments. The devices were analyzed by the characterization of cross-bridge structures. Results show that the room temperature Hall mobilities and the low-frequency excess noise levels of the devices vary systematically with the light intensity and the concentration of the solution. These results are consistent with the changes of surface roughness. It is found that when the devices were treated by ultrasonic agitation after etching process, the corresponding noise level decreased. A Lorentzian bump originating from the generation-recombination process is observed at low temperature range and its thermal activation energy is about 86.2meV. In addition, it is observed that the low-frequency excess noise of the devices fabricated by optimal PEC etching condition is about half of an order of magnitude lower than that of the devices fabricated by inductively coupled plasma (ICP) dry etching technique. The room temperature Hooge parameter of the device fabricated under the optimal conditions is found to be 2 x 10⁻³ compared to a value of 9 x 10⁻³ for the device fabricated by conventional ICP etching technique. The experimental results demonstrate that PEC wet etching can be effectively utilized as an alternative technique for the fabrication of GaN devices.

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